Method and Kit for Detecting Lead in a Solid Sample

ABSTRACT

A method and kit for detecting the presence of lead in a solid sample is disclosed. The solid sample is brought into contact with a nonaqueous solution of a dithiocarbamate or quinoline capable of forming a gel upon contact with lead, with the formation of a gel showing that lead is present in the solid sample.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 15/307,021, filed Oct. 27, 2016, which claims the benefit of U.S. provisional patent application No. 61/987,786, filed May 2, 2014, incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENTAL INTEREST

This invention was made with government support under N00014-12-1-0604 awarded by the U.S. Navy, Office of Naval Research. The Government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to the detection of lead in a solid sample, for example, a paint chip. More particularly, the present invention relates to a method and kit for the detection of lead in a solid sample wherein gelation of a reagent solution demonstrates the presence of lead in the solid sample.

BACKGROUND OF THE INVENTION

Lead has been used for centuries in coatings, for example, as a pigment in paints, a corrosion inhibitor, and a drier in paints and varnishes. The primary reasons for using lead in coatings are properties such as durability of a finished coating, the broad spectrum of available colors using lead compounds, and corrosion, water, and weather resistance.

When lead-containing coatings age, the coatings deteriorate and consequently create lead-containing or lead-contaminated dust. Similarly, when lead-containing coatings are disturbed by cutting, drilling, sanding, or other methods commonly employed to remove building materials, lead-containing dust is released from the disturbed coating, and the dust can readily disseminate and contaminate large areas. This is a well-known public health hazard because lead-contaminated dust has been identified as a significant health hazard, particularly to children. Lead-based paints therefore were banned in 1978, but it is estimated that greater than 20 million homes may still contain lead-based coatings.

Lead contamination is not limited to paint and related products because lead has been used for decades in a wide variety of applications, such as building construction materials (roofing material, cladding, flashing, gutters and gutter joints, and on roof parapets), ammunitions, lead-acid car batteries, weights, fusible alloys, radiation shields (lead glass), and cosmetics. The use of lead in paint (wall paint, oil and water-based paint in art, paint used in toys), fuel, pipe, and plumbing material, solder for cars and pesticides has been greatly reduced over the last several decades because of the danger of lead poisoning.

Lead poisoning is a medical condition caused by increased levels of lead in the body. Exposure and accumulation to lead and lead-containing chemicals can occur through inhalation, ingestion, and/or dermal contact. Lead affects several organs in the body, and especially the nervous system. Lead also adversely effects bones (weakness in fingers, wrists, or ankles), teeth, kidneys (nephropathy and colic-like abdominal pain), the cardiovascular (blood pressure), immune, and reproductive (reduced fertility in males) systems, and can cause miscarriage in pregnant females. The adverse effects of lead on the nervous system is more pronounced in children than in adults.

Lead poisoning can be prevented by detecting lead or lead-containing compounds, and reducing or avoiding exposure to lead. To address the issue of the health risks from lead-containing dust, the United States Environmental Protection Agency (USEPA) promulgated regulations governing the appropriate methods and procedures to be used during remodeling and renovation of residential housing. These regulations stipulate that, for all housing built before 1978, the surfaces to be disturbed during renovation or remodeling must be checked for the presence of lead in the surface coatings. There are several ways lead testing can be accomplished, including Atomic Absorption spectroscopy, X-Ray Fluorescence, and chemical tests.

Atomic Absorption Spectroscopy and X-Ray Fluorescence involve sophisticated instrumentation and are expensive procedures. Several commercially-available, do-it-yourself lead test kits are available, but these kits are not quantitative and often lack credibility because they lead to false positive results. The chemical tests also often lack selectivity because test samples often contain additional compounds that can interfere with the assay for lead. Such additional compounds can include chromium, mercury, cadmium, zinc, barium, nickel, cobalt, copper, antimony, bismuth, titanium, and other metals present in pigments.

In addition, in order to achieve the different benefits of lead in various types of coatings, different forms of lead and lead-containing compounds have been incorporated into the coatings. Lead compounds that have been used in coatings include, but are not limited to lead sulfate, lead chromate, lead monoxide (litharge), lead silicate, lead sulfate blue basic, lead linoleate, lead naphthenate, and lead carbonate. While each type of lead compound has certain properties that differentiate the compound from the other lead compounds, the commonality of lead in each compound renders each compound a health risk. It is important that a detection method has the capability of detecting each type of lead compound that may be present in the solid sample. For example, some chemical assays fail to detect lead chromate.

In addition, although chemical tests to detect lead are commercially available, the tests may not properly detect lead levels at thresholds promulgated by regulatory authorities, e.g., lead equal to or exceeding 1.0 milligram per square centimeter (mg/cm²) or 0.5 percent by weight (equivalent to 5,000 parts per million or ppm). In such cases, either false positive or false negative results may arise, which in the case of a false negative can lead to health issues and in the case of a false positive to unneeded and expensive remediation measures.

A need therefore still exists in the art for a fast and accurate method and test kit to detect levels of lead in a solid sample, while avoiding false positive and false negative assay results. A method and test kit that can be used by a homeowner or in the field, without the need to submit samples for testing or to require substantial user training, also is an unmet need in the art.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the reaction which forms a gel between lead and a dithiocarbamate.

FIG. 2 illustrates the selectivity of gel formation upon contact with a lead-containing sample vs. other metal-containing sample.

FIG. 3 illustrates various nonlimiting nonaqueous solvents used in the method of the present invention.

SUMMARY OF THE INVENTION

The present invention is directed to a testing kit and method of assaying a solid sample for the presence of lead. The method accurately determines the presence of lead in the solid sample without interference from other metals that may be present. In one embodiment, the kit includes (a) a dithiocarbamate or quinoline compound that is capable of complexing with lead to form a gel and (b) a nonaqueous solvent. In other embodiments, the kit further contains a resealable container in which to conduct the assay.

Another aspect of the invention is to provide a method of determining the presence or absence of lead in a solid by adding a small sample of the solid to a nonaqueous solution of a dithiocarbamate or quinoline capable of complexing with lead, then visually observing the resulting mixture for the formation of a gel. The dithiocarbamate or quinoline is capable of complexing with additional metals, but the resulting complexes do not form a gel.

These and other aspects of the invention will become apparent from the following nonlimiting detailed description the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Six criteria for a lead detection method have been identified. The method should be (a) capable of detecting lead concentrations of 1 mg/cm², (b) non-hazardous, (c) suitable for use as a nondestructive or minimally destructive field method, (d) suitable for use by non-technical personnel, (e) sufficiently reliable, precise, and accurate, and (f) rapid. The present invention is provides an inexpensive, more reliable lead detector that meets each of these six criteria.

The present invention provides a portable test kit to detect lead in solid samples, such as paints. The test kit contains a dithiocarbamate that gels upon binding to lead, either immediately or within a short reaction time period.

The method provides an unambiguous visual change in the physical properties/appearance of the dithiocarbamate solution upon binding to lead. The present method enables the detection of lead by naked eye with no additional instrumentation or training. The method has low critical gel concentration facilitating detection of lower analyte concentrations, i.e., as low as about 200 ppm of lead.

The present test kit can comprise a container in which the assay for lead is performed. In some embodiments, the container is optional. In this case, the individual performing the lead assay can provide a suitable container to perform the present method. The container is manufactured from a transparent material, which allows an easy visual inspection for the formation of a gel. The container therefore can be prepared from glass or a transparent plastic, such as a polycarbonate, acrylate, or urethane, for example. The shape of the container is not limited, but often is in the shape of a cylinder. The container typically has a volume of about 3 to about 25 ml, and typically, about 4 to about 10 ml, which facilitates use of the test kit in the field.

An important feature of the present method and kit is to utilize a dithiocarbamate or quinolone capable of complexing with lead to form an insoluble gel in a nonaqueous solvent. Useful dithiocarbamates have a structural formula (I):

wherein n is an integer 1 or 2; m is an integer 1 or 2; R¹ and R², individually, are selected from the group consisting of C₁-C₁₀ linear alkyl, C₃-C₁₀ branched alkyl, C₂₋₆linear alkenyl, C₂₋₆ branched alkenyl, C₃-C₁₀ cycloalkyl, substituted and unsubstituted aryl, substituted and unsubstituted (CH₂)₁₋₃aryl, and

or R¹ and R² are taken together with the nitrogen atom to which they are attached to form a five or six membered ring, optionally fused to an aryl ring; and Y is an element selected from Groups IA and IIA of the periodic table.

In preferred embodiments, R¹ and R², the same or different, are substituted or unsubstituted aryl, substituted or unsubstituted (CH₂)₁₋₃aryl, e.g., sodium dibenzyldithiocarbamate, or C₂₋₆linear alkenyl, e.g., sodium diallyldithiocarbamate..

Compounds of formula (I) include, but are not limited to, sodium dimethyldithiocarbamate (CAS #128-04-1), sodium diethyldithiocarbamate (CAS #148-18-5), sodium dibenzyldithiocarbamate (CAS #55310-46-8), sodium diallyldithiocarbamate, sodium allylbenzyldithiocarbamate,

and mixtures thereof.

In some embodiments, R¹ is benzyl, and R² is selected from the group consisting of para-methylbenzyl, 2,4,6-trimethylbenzyl, and

In another embodiment, a quinoline capable of complexing with lead is used to form an insoluble gel in a nonaqueous solvent. One nonlimiting quinoline has the structure

The dithiocarbamate or quinoline is used in a sufficient amount to complex with lead in the solid sample, for example, up to the solubility limit in the nonaqueous solvent used in the assay method. Typically, the dithiocarbamate or quinoline is present in a nonaqueous solvent in an amount of about 0.05 to about 5%, preferably about 0.1 to about 3%, and more preferably about 0.25 to about 1%, by weight, of the nonaqueous solution.

In addition to the dithiocarbamate or quinoline, the assay method utilizes a nonaqueous solvent. The identity of the nonaqueous solvent is limited solely by an ability to solubilize the dithiocarbamate or quinoline in a sufficient amount to complex with lead in a solid sample. The time required to form a lead-containing gel after contact between a solid sample and the dithiocarbamate or quinoline can vary depending upon the nonaqueous solvent used in the assay. Selection of an optimum solvent, and selection of the concentration of dithiocarbamate or quinoline, therefore can be readily determined by persons skilled in the art after considering the time requirements desired to obtain assay results. In preferred embodiments, the amount of water present in the nonaqueous solvent is less than 50% by volume, preferably less than 10%, by volume.

Examples of useful classes of nonaqueous solvents include, but are not limited to, alcohols, esters, glycols, glycol ethers, aliphatic and aromatic hydrocarbons, chlorinated solvents, and mixtures thereof. Specific nonlimiting examples of solvents include, but are not limited to, C₁₋₆alcohols, e.g., methanol, ethanol, propyl alcohol, and butyl alcohol, including isomers thereof; monoC₁₋₄alkyl ethers of ethylene glycol and propylene glycol; acetone; methyl ethyl ketone; isophorone; dichloromethane; chloroform; ethyl acetate; 2-methoxyethanol; DMF; DMSO; THF; acetonitrile; kerosene; mineral spirits; xylene; toluene; and mixtures thereof. Also useful are standard solvents used in the paint industry, e.g., paint thinner and turpentine.

In accordance with the present invention, a solid sample is assayed for the presence of lead by providing a reagent solution comprising a dithiocarbamate or quinoline dissolved in a nonaqueous solvent. The reagent solution is placed in a transparent container and a sample of the solid is added to the solution. Alternatively, the reagent solution can be added to a transparent container containing the solid sample.

The resulting mixture optionally is shaken, then is allowed to stand uninterrupted. The container then is visually examined for the formation of a gel, as illustrated in FIG. 1. Gel formation occurs within 10 minutes, typically within 5 minutes, and usually within 2 minutes. The detection of lead in a solid sample therefore is rapid and can be performed at home or in the field.

The complexing reaction between lead and the dithiocarbamate or quinoline to form a gel occurs at ambient temperature, e.g., about 20 to 30° C. To facilitate and speed gel formation, the reaction mixture can be heated above ambient temperature, for example, up to about 50° C., which temperature is limited by the identity and boiling point of the nonaqueous solvent used in the assay.

Various embodiments are envisioned for practice of the invention at home or in the field. In one embodiment, a kit comprising a ready-to-use reagent solution of a dithiocarbamate or quinoline in a non-aqueous solvent in a transparent container is provided. The user then merely adds a solid test sample to the container, and the container is visually observed for a response.

In other embodiments, the kit comprises the dithiocarbamate or quinoline either neat or as a concentrated solution in a nonaqueous solvent. In use, the concentrated dithiocarbamate or quinoline either is added to a container contained in a kit or is added to a container provided by the user, then the user adds a sufficient amount of nonaqueous solvent to the concentrated solution to provide a reagent solution for the lead assay.

In various embodiments, the kit contains an instruction sheet for proper use of the kit, e.g., amount of solid sample to add to the reagent solution or amount and identity of solvent to add to the neat or concentrated solution of dithiocarbamate or quinoline.

EXAMPLE 1 In Situ Gel Formation

An 8 mL vial was charged with sodium N,N-dibenzyldithiocarbamate dissolved in methanol (0.8 mL), and the resulting mixture was heated to form a solution. A suspension of PbO in water (0.2 mL) then was added to the resulting solution. The vial was sealed, the resulting mixture was heated until homogeneous, then cooled to ambient temperature after about 20 seconds of sonication in a water bath at ambient temperature to form a gel.

In the above examples, sonication was used to speed formation of the lead-containing gel. In practical use at home or in the field, sonication is not required because the gel forms spontaneously over a slightly longer time period.

In addition to the PbO used in the examples, a lead-containing gel also forms when the solubilized dithiocarbamate forms complexes with other lead salts (e.g., PbCrO₄, PbCO₃, Pb(NO₃)₂, and Pb(OAc)₂. The solubilized dithiocarbamite also forms complexes with other metals, but the resulting complex does not form a gel. The present method therefore is specific for lead.

EXAMPLE 2

To illustrate the selectivity of the present method and kit, a series of experiments was performed. In particular, an 8 mL vial was charged with sodium N,N-dibenzyldithiocarbamate (3.3 mg, 0.012 mmol) and acetone (0.5 mL), which then was shaken to dissolve the dithiocarbamate. Then, a metal acetate salt (0.006 mmol) suspended in acetone (0.5 mL) was added to the dithiocarbamate solution. The vial was capped and heated to dissolve the metal acetate salt, and the mixture then was cooled to room temperature. FIG. 2 shows that the dithiocarbamate forms a complex with a metal other than lead, but does not form a gel. The kit therefore is selective for the detection of lead.

EXAMPLE 3

To illustrate that a variety of nonaqueous solvents can be used in the present method and kit, experiments were performed using a water-immiscible solvent and a water-miscible solvent were conducted. These solvents are known to dissolve latex- and oil-based paints. For example, an 8 mL vial was charged with sodium N,N-dibenzyldithiocarbamate (4.8 mg, 0.016 mmol), oil-based exterior paint (0.05 mL), and commercial paint thinner (0.5 mL). The resulting mixture was shaken to form a solution. Then, Pb(OAc)₂ (5.0 mg, 0.015 mmol) suspended in paint thinner (0.5 mL) was added to the solution in the vial. The vial was capped, heated to dissolve the lead salt, then cooled to room temperature to form a gel. In another example, an 8 mL vial was charged with sodium N,N-dibenzyldithiocarbamate (4.4 mg, 0.015 mmol), latex-based paint (0.05 mL), and acetone (0.5 mL). The resulting mixture was shaken to form a solution. Then, Pb(OAc)₂ (5.6 mg, 0.017 mmol) suspended in acetone (0.5 mL) was added to the solution in the vial. The vial was capped and heated to dissolve the lead salt, then cooled to room temperature. Upon cooling, a gel was formed. The results are illustrated in FIG. 3. The center vial of FIG. 3 shows that the present method is operative when both a latex- and oil-based paint are present in the same sample.

To demonstrate the present invention, various dithiocarbamates and quinolines were tested for an ability to complex with lead and provide a gel. In particular, compounds 1a-h and 2a,b were prepared in high yield and evaluated for an ability to gel lead.

TABLE 1

R R′ 1a Et iPr 1b Et Et 1c Bu Bu 1d Bn Bn 1e

1f Bn

1g Bn PMB 1h Bn

Bn = benzyl; PMB = para-methoxybenzyl 2a; R″ = Me 2b; R″ = H

As shown in Tables 2-4, compounds 1d-h and 2b formed gels in solvents relevant to sensing lead paint, including paint thinner, acetone, and methyl ethyl ketone. Rheological studies of each gel revealed that the storage modulus (G′) was at least 1 order of magnitude higher than the loss modulus (G″), which is characteristic of small-molecule gels. Scanning electron microscopy revealed that the morphologies consisted of high aspect ratio fibers (about 0.1-3 μm in diameter), which is also characteristic of molecular gels.

The present invention provides a simple to use method and a kit to assay solid samples for the presence of lead. The method and kit can be used at home or on site in the field to detect lead in paint during remodeling or renovation of older houses or buildings by general contractors and homeowners; lead in children's toys and play areas; lead in drinking water; lead in pipes, solders, and plumbing; and lead in cosmetics.

The present method and test kit provide advantages over existing assays including a simple and easy to use technology, an unambiguous detection of lead through a visible gel formation, no equipment or training to interpret test results, and an increased selectivity compared to existing lead assays.

Gel screening 1a-1h and 2a-2b. A known amount of lead complex (about 1-10 mg) was weighed into an 8 mL vial. Then, 0.3-0.5 mL of indicated solvent was added and the vial was capped. If the material did not immediately dissolve, the mixture was heated and/or sonicated to form a solution. If the compound did not solubilize upon heating, additional solvent was added until all material dissolved upon heating. The vial was heated, and then cooled with approximately 20 seconds (1a-1d) to 30 seconds (2a-2b) of sonication in a water bath (near ambient temperature). After standing undisturbed at room temperature for at least 10 minutes, the vial was inverted to test for gelation.

TABLE 2 Gel screening for 1a-1h^(a) Solvent 1a 1b 1c 1d 1e 1f 1g 1h acetone sol ppt sol gel sol gel gel ppt EtOAc ppt ppt sol gel sol sol gel gel MeOH ppt ppt cryst gel gel gel gel ppt toluene sol ppt sol gel sol sol sol sol THF sol sol sol gel sol sol sol sol iPrOH ppt ppt cryst gel gel gel gel ppt CH₂Cl₂ sol sol sol gel sol sol sol sol H₂O ppt ppt ppt ppt ppt ppt ppt ppt EtOH ppt ppt sol gel gel gel gel ppt hexanes ppt ppt sol ppt ppt ppt ppt ppt paint ppt ppt sol gel gel gel gel ppt thinner MEK^(b) sol cryst sol gel sol sol gel gel ^(a)sol = solution; ppt = precipitate; cryst = crystals. ^(b)MEK = methyl ethyl ketone.

TABLE 3 Gel screening for 2a-b.^(a) Solvent 2a 2b acetone ppt ppt EtOAc ppt ppt 1M NaOH ppt gel EtOH/1M NaOH^(b) ppt gel MeOH/1M NaOH^(b) ppt gel H₂O ppt ppt MeOH/H₂O^(c) p p ^(a)sol = solution; ppt = precipitate. ^(b)1:1 mixture of solvents. ^(c)2:1 mixture of solvents.

Determining the critical gel concentration (cgc) of 1d-1f. The cgc was determined by adding a known amount of gelator (1d-1f) (about 1-10 mg) into an 8 mL vial. To the vial, 0.3 mL solvent was added. The vial was capped, heated to dissolve the solid, and cooled with approximately 20 seconds of sonication in a water bath (near ambient temperature). After standing undisturbed at room temperature for at least 10 minutes, the vial was inverted to test for gelation. If the resulting gel was stable to inversion, additional solvent (0.1 mL) was added, and the heating, sonication, and resting steps repeated. Additional 0.1 mL increments of solvent were added until the gel was no longer stable to inversion. The reported cgcs are averages of three experiments.

Determining the cgc of 2b. A known amount of compound 2b (about 5-10 mg) was placed in an 8 mL vial. Then, the indicated solvent (0.5-1.0 mL) was added. The vial was capped and heated to dissolve the solid, then cooled with about 30 seconds of sonication in a water bath (near ambient temperature). After standing undisturbed at room temperature for at least 10 minutes, the vial was inverted to test for gelation. If the resulting gel was stable to inversion, additional solvent (0.1 mL) was added, and the heating, sonication, and resting steps repeated. Additional 0.1 mL increments of solvent were added until the gel was no longer stable to inversion. The reported cgcs are averages of three experiments.

In an alternative gelation procedure, a known amount of compound 2b was weighed out and placed in an 8 mL vial with 0.5 mL of EtOH. Then, 50 μL of acetic acid was added and the vial was capped and shaken to dissolve. Next, 1 mL of 1M NaOH was added in 0.1 mL increments. Finally, the vial was capped and heated to dissolve, then allowed to cool with about 30 seconds of sonication in a water bath (near ambient temperature). The cgc of 2b was determined by decreasing the amount weighed out until gelation no longer occurred.

TABLE 4 Cgcs for 1d-1h and 2b compoun acetone ethyl acetate paint thinner basic 1d 1.84 ± 0.07 1.29 ± 0.09 0.77 ± 0.13 — mM 1e — — 4.12 ± 0.04 — 1f 27.81 ± 4.30 — 1.11 ± 0.04 — 1g 3.79 ± 0.33 5.17 ± 0.48 1.46 ± 0.08 — mM 1h — 3.83 ± 1.13 — — 2b — — — 16.1 ± 0.86 mM^(a) 2b — — — 13.9 ± 0.30 ^(a)1:1 mixture EtOH/1M NaOH. b) Using the alternative gelation procedure described above.

To further demonstrate the selectivity of the present method for lead, sodium dibenzylcarbamate was added to solutions containing acetate salts of Ni²⁺, Ca²⁺, Cd²⁺, Ba²⁺, Cu²⁺, Zn²⁺, Mn²⁺, and Fe²⁺, which did not result in gels as shown in Table 5.

It is theorized that the dithiocarbamate formed complexes with these metals, but the resulting products were not gelators either due to unfavorable geometries or intermolecular interactions. Importantly, Pb-triggered gelation was unaffected by high concentrations of these other metal salts when excess ligand was added as shown in Table 6.

General Procedure

A known amount of metal acetate salt (Ni(OAc)₂.4H₂O, Ca(OAc)₂, Cd(OAc)₂.2H₂O, Ba(OAc)₂, Cu(OAc)₂.2H₂O, Zn(OAc)₂.2H₂O, Mn(OAc)₂, Fe(OAc)₂, or Pb(OAc)₂.3H₂O) in 1 mL paint thinner in an 8 mL vial, was treated with a solution of 4.1 mg 3 in 0.2 mL MeOH. The vial was capped and sonicated for 20 seconds to ensure complete mixing. Once mixing appeared complete, the vial was heated to dissolve the solid, and subsequently cooled with approximately 20 seconds of sonication in an water bath (near ambient temperature). After standing undisturbed at room temperature for about 10-15 minutes, the vial was inverted to test for gelation. The heating and sonication steps were repeated if no gel formed.

TABLE 5 Testing gelation with various metal acetate salts M(OAc)₂•xH₂O mg gel? appearance Ni(OAc)₂•4H₂O 1.7 no green ppt Ca(OAc)₂ 1.2 no white ppt Cd(OAc)₂•2H₂O 1.4 no clear solution Ba(OAc)₂ 1.7 no clear solution Cu(OAc)₂•2H₂O 1.4 no brown solution Zn(OAc)₂•2H₂O 1.9 no clear solution Mn(OAc)₂ 2.2 no brown/purple ppt Fe(OAc)₂ 1.1 no brown/red ppt Pb(OAc)₂•3H₂O 2.7 yes white gel

General Procedure

A solution of Pb(OAc)₂.3H₂O (2.7 mg in 0.1 mL MeOH) was added to an 8 mL vial containing the other M(OAc)₂.xH₂O (Ni(OAc)₂.4H₂O, Cu(OAc)₂.2H₂O, Zn(OAc)₂.2H₂O, Mn(OAc)₂, or Fe(OAc)₂) in 1 mL paint thinner. Subsequently 3 (4.1 mg in 0.1 mL MeOH) was added. The vial was capped and sonicated for 20 seconds to ensure complete mixing. Once mixing appeared complete, the vial was heated to dissolve the solid, and subsequently cooled with approximately 20 seconds of sonication in a water bath (near ambient temperature). After standing undisturbed at room temperature for about 10-15 minutes, the vial was inverted to test for gelation. The heating and sonication steps were repeated if no gel formed/no reaction occurred.

TABLE 6 Testing gelation with lead in the presence of other metal salts M(OAc)₂•xH₂O mg gel? appearance — — yes white gel Ni(OAc]₂•4H₂O 1.7 yes^(b) green gel Cu(OAc)₂•2H₂O 1.3 yes^(c) brown/white gel Zn(OAc)₂•2H₂O 1.6 yes white gel Mn(OAc)₂ 1.7 yes white gel with red/purple specs Fe(OAc)₂ 1.2 yes yellow gel a) All vials contain Pb(OAc)₂•3H₂O. ^(b)An additional 2 mg 3 in 0.05 mL MeOH was added to form a stable gel. ^(c)An additional 5.1 mg 3 in 0.1 mL MeOH was added to form a stable gel.

Synthetic Procedures Materials

All reagent grade materials and solvents were purchased from Sigma Aldrich, Acros Organics, or TCI. The paint thinner used was Klean-Strip paint thinner made with mineral spirits. Paints used were as follows: black oil-based paint: Rust-Oleum Professional, V7579 Gloss Black, High performance enamel; pink latex-based paint: Valspar Satin Berry Twist 530832, Spring 2014; white oil-based paint: Rust-Oleum, 7792 Gloss White, Protective Enamel. All alkyl amines and carbon disulfide were distilled prior to use. Methanol was dried over activated molecular sieves under N₂ overnight. All other compounds were used without further purification unless otherwise noted. Compounds S1-S3, 1a-h, 2a-b, and 3 were prepared from modified literature procedures. Throughout this document water refers to deionized water, unless otherwise noted.

General Procedures

¹H and ¹³C NMR spectra for all compounds were acquired in deuterated solvents on either a Varian VNMRS 700 operating at 700 and 176 MHz, a Varian VNMRS 500 operating at 500 and 126 MHz, or a Varian MR400 operating at 400 MHz. The chemical shift data are reported in units of 6 (ppm) relative to tetramethylsilane and referenced by residual protic solvent. An asterisk was used to indicate residual H₂O in all spectra while double bars are used to indicate peaks that have been truncated. Multiplicities are reported as follows: singlet (s), doublet (d), triplet (t), quartet (q), septet (sept), doublet of doublets (dd), triplet of quartets (tq), doublet of doublet of triplets (ddt) and multiplet (m).

High-Resolution Mass Spectrometry (HRMS) data were obtained on a Micromass AutoSpec Ultima Magnetic Sector mass spectrometer via electron impact ionization on a desorption probe or via electrospray ionization.

Scanning Electron Microscopy (SEM) images were obtained on a Hitachi S3200N SEM using a 15-kV accelerating voltage or a FEI NOVA Nanolab Dualbeam Workstation with a Schottky field emitter operated at 10-20 kV accelerating voltage.

Powder X-Ray Diffraction data were collected at ambient temperature using a Bruker D8 Advance diffractometer with a LynxEye detector using graphite monochromated Cu-Kα radiation (1.5406 Å) operating at 40 kV, 40 mA and on a Rigaku SmartLab diffractometer in reflection mode using point-focus Cu-Kα radiation (λ=1.5425035) operating at 40 kV, 44 mA and equipped with a Pilatus 100K/R 2-dimensional detector set at a nominal sample to detector distance of 200 mm. The samples were analyzed on glass microscope slides with wells.

Materials Studio—All calculations were performed with Materials Studio 6.0 by Accelrys Software Inc. The aspect ratios (AR) were calculated by dividing the longest distance by the shortest distance within a crystal.

Synthetic Procedures A. Synthesis of Amines

N-Benzyl-1-(furan-2-yl)methanamine (S1). 2-Furaldehyde (300 μL, 3.63 mmol) and benzylamine (360 μL, 3.30 mmol) were combined in dry MeOH (9 mL) and stirred under N₂ for 18 hours. The solution was then treated with NaBH₄ (279 mg, 7.38 mmol) in small portions, and stirred under N₂. After about 1 hour, no starting material was visible by TLC. The reaction was carefully quenched with H₂O (10 mL). Methanol (MeOH) was removed via rotary evaporation, and the aqueous residue extracted with ethyl acetate (EtOAc) (3×10 mL). The combined organic layers were dried over magnesium sulfate (MgSO₄), filtered, and the solvent removed via rotary evaporation. The resulting oil was purified by flash column chromatography, eluting with 14% to 20% EtOAc in hexanes to give a clear oil (518 mg, 84%). HRMS (ESI): Calcd for C₁₂H₁₄NO⁺, 188.1070. Found, 188.1066.

N-Benzyl-1-(4-methoxyphenyl)methanamine (S2). 4-Methoxybenzaldehyde (740 μL, 6.08 mmol) and benzylamine (600 μL, 5.49 mmol) were combined in dry MeOH (15 mL) and stirred under N₂ for 18 h. The solution was then treated with sodium borohydride (NaBH₄) (420. mg, 11.1 mmol) in small portions, and stirred under nitrogen (N₂). After aboutl hour, no starting material was visible by TLC. The reaction was carefully quenched with H₂O (10 mL). MeOH was removed via rotary evaporation, and the aqueous residue extracted with CH₂Cl₂ (3×10 mL). The combined organic layers were dried over MgSO₄, filtered, and the solvent removed via rotary evaporation. The resulting oil was purified by flash column chromatography, eluting with 20% to 33% EtOAc in hexanes to give a clear oil (1068 mg, 86%). HRMS (ESI): Calcd for C₁₅H₁₈NO⁺, 228.1383. Found, 228.1380.

N-Benzyl-1-mesitylmethanamine (S3). Mesitaldehyde (890 μL, 6.00 mmol) and benzylamine (600 μL, 5.49 mmol) were combined in dry MeOH (15 mL) and stirred under N₂ for 19 hours. The solution was then treated with NaBH4 (420. mg, 11.1 mmol) in small portions, and stirred under N₂. After aboutl hour, no starting material was visible by TLC. The reaction was carefully quenched with H₂O (10 mL). MeOH was removed via rotary evaporation, and the aqueous residue extracted with EtOAc (3×15 mL). The combined organic layers were dried over MgSO₄, filtered, and the solvent removed via rotary evaporation. The resulting oil was purified by flash column chromatography, eluting with 14% EtOAc in hexanes to give a light yellow oil (1059 mg, 81%). HRMS (ESI): Calcd for C₁₇H₂₂N⁺, 240.1747. Found, 240.1742.

B. Synthesis of Lead Complexes

N²,N⁶-Diethyl-N²,N⁶-diisopropyl-1λ³,3,5λ³,7-tetrathia-4-plumbaspiro[3.3]hepta-1,5-diene- 2,6-diamine (1a). N-Isopropyl-N-ethyl-amine (150 μL, 1.24 mmol) and carbon disulfide (80 μL, 1.3 mmol) were added to a round-bottom flask equipped with a stirbar and dissolved in EtOH (14 mL). Lead(II) oxide (138 mg, 0.618 mmol) was added. The flask was sealed and stirred under N₂ for 16 hours, forming a white precipitate. Then, CH₂Cl₂ (15 mL) was added to dissolve the precipitate and was subsequently filtered through Celite, washing with CH₂Cl₂ (about 50 mL) to remove unreacted PbO. The filtrate was concentrated via rotary evaporation, yielding an off-white solid (310 mg, 94%). HRMS (EI): Calcd for C₁₂H₂₄N₂PbS₄ ⁺, 532.0583. Found, 532.0570.

N²,N²,N⁶,N⁶-Tetraethyl-1λ³,3,5λ³,7-tetrathia-4-plumbaspiro[3.3]hepta-1,5-diene-2,6-diamine (1b). N,N-Diethylamine (130 μL, 1.26 mmol) and carbon disulfide (80 μL, 1.3 mmol) were added to a round-bottom flask equipped with a stirbar and dissolved in EtOH (14 mL). Lead(II) oxide (140. mg, 0.627 mmol) was added. The flask was sealed and stirred under N₂ for 16 hours, forming a precipitate. Then, CH₂Cl₂ (20 mL) was added to dissolve the precipitate and was subsequently filtered through Celite washing with CH₂Cl₂ (about 75 mL) to remove unreacted lead oxide (PbO). The filtrate was concentrated via rotary evaporation, yielding a gray solid (272 mg, 86%). HRMS (EI): Calcd for C₁₀H₂₀N₂PbS₄ ⁺, 504.0270. Found, 504.0286.

N²,N²,N⁶,N⁶-Tetrabutyl-1λ³,3,5λ³,7-tetrathia-4-plumbaspiro[3.3]hepta-1,5-diene-2,6-diamine (1c). N,N-Dibutylamine (210 μL, 1.25 mmol) and carbon disulfide (80 μL, 1.3 mmol) were added to a round-bottom flask equipped with a stirbar and dissolved in EtOH (14 mL). Lead(II) oxide (140. mg, 0.627 mmol) was added. The flask was sealed and stirred under N₂ for 16 hours, forming a white precipitate. Then, CH₂Cl₂ (20 mL) was added to dissolve the precipitate and was subsequently filtered through Celite washing with CH₂Cl₂ (about 50 mL) to remove unreacted PbO. The filtrate was concentrated via rotary evaporation, yielding a yellow solid (372 mg, 97%). HRMS (EI): Calcd for C₁₈H₃₆N₂PbS₄ ⁺616.1522. Found, 616.1545.

N²,N²,N⁶,N⁶-Tetrabenzyl-1λ³,3,5λ³,7-tetrathia-4-plumbaspiro[3.3]hepta-1,5-diene-2,6- diamine (1d). N,N-Dibenzylamine (100 μL, 0.520 mmol) and carbon disulfide (50 μL, 0.83 mmol) were added to a round-bottom flask equipped with a stirbar and dissolved in EtOH (6 mL). Lead(II) oxide (58.3 mg, 0.261 mmol) was added. The flask was sealed and stirred under N₂ for 16 hours, forming a white precipitate. Then, CH₂Cl₂ (50 mL) was added to dissolve the precipitate and was subsequently filtered washing with CH₂Cl₂ (about 150 mL) to remove unreacted PbO. The filtrate was concentrated via rotary evaporation, yielding a white solid (182.0 mg, 93%). HRMS (EI): Calcd for C₃₀H₂₈N₂PbS₄ ⁺, 752.0896. Found, 752.0898.

N²,N²,N⁶,N⁶-Tetraallyl-1λ³,3,5λ³,7-tetrathia-4-plumbaspiro[3.3]hepta-1,5-diene-2,6-diamine (1e). A solution of NaOH (65 mg, 1.6 mmol) in H₂O (0.1 mL) and MeOH (6 mL) was stirred at 0° C. for 5 minutes prior to adding diallylamine (200 μL, 1.62 mmol). After 10 min, carbon disulfide (100 μL, 1.62 mmol) was added. The reaction was allowed to come to room temperature and stirred for 18 hours. The solvent was removed to give a yellow oil.

A portion of the crude product (50.0 mg, 0.256 mmol) was dissolved in EtOH (3 mL) and treated with PbO (28.6 mg, 0.128 mmol). The flask was sealed and stirred under N₂ for 16 hours, forming a white precipitate. Then, CH₂Cl₂ (5 mL) was added to dissolve the precipitate and was subsequently filtered through Celite, washing with CH₂Cl₂ (about 50 mL) to remove unreacted PbO. The filtrate was concentrated via rotary evaporation, yielding a white solid (67.8 mg, 96%). The product is not stable in CH₂Cl₂ so exposure was minimized. HRMS (EI): C₁₄H₂₀N₂PbS₄ ⁺, 552.0270. Found, 552.0276.

N²,N⁶-Dibenzyl-N²,N⁶-bis(furan-2-ylmethyl)-1λ³,3,5λ³,7-tetrathia-4-plumbaspiro[3.3]hepta- 1,5-diene-2,6-diamine (1f). Amine Si (99.7 mg, 0.530 mmol) and carbon disulfide (60 μL, 1.1 mmol) were added to a round-bottom flask equipped with a stirbar and dissolved in EtOH (6 mL). Lead(II) oxide (59.3 mg, 0.266 mmol) was added. The flask was sealed and stirred under N₂ for 21 hours, forming a white precipitate. Then, CH₂Cl₂ (about15 mL) was added to dissolve the precipitate and was subsequently filtered through Celite, washing with CH₂Cl₂ (about 50 mL) to remove unreacted PbO. The filtrate was was concentrated via rotary evaporation, yielding a white solid (187 mg, 97%). HRMS (EI): C₂₆H₂₄N₂O₂PbS₄₊, 732.0481. Found, 732.0468.

N²,N⁶-Dibenzyl-N²,N⁶-bis(4-methoxybenzyl)-1λ³,3,5λ³,7-tetrathia-4-plumbaspiro[3.3]hepta-1,5-diene-2,6-diamine (1g). Amine S2 (150. mg, 0.660 mmol) and carbon disulfide (60 μL, 0.99 mmol) were added to a round-bottom flask equipped with a stirbar and dissolved in EtOH (6 mL). Lead(II) oxide (73.9 mg, 0.330 mmol) was added. The flask was sealed and stirred under N₂ for 18 hours, forming a white precipitate. Then, CH₂Cl₂ (about 20 mL) was added to dissolve the precipitate and was subsequently filtered through Celite, washing with CH₂Cl₂ (about 100 mL) to remove unreacted PbO. The filtrate was concentrated via rotary evaporation, yielding a white solid (257 mg, 96%). HRMS (EI): C₃₂H₃₂N₂O₂PbS₄ ⁺, 812.1107. Found, 812.1133.

N²,N⁶-Dibenzyl-N²,N⁶-bis(2,4,6-trimethylbenzyl)-1λ³,3,5λ³,7-tetrathia-4-plumbaspiro[3.3]hepta-1,5-diene-2,6-diamine (1 h). Amine S3 (150. mg, 0.630 mmol) and carbon disulfide (60 μL, 1.0 mmol) were added to a round-bottom flask equipped with a stirbar and dissolved in EtOH (6 mL). Lead(II) oxide (70.0 mg, 0.310 mmol) was added. The flask was sealed and stirred under N₂ for 21 hours, forming a white precipitate. Then, CH₂Cl₂ (about 25 mL) was added to dissolve the precipitate and was subsequently filtered through Celite, washing with CH₂Cl₂ (about 100 mL) to remove unreacted PbO. The filtrate was concentrated via rotary evaporation, yielding a white solid that was a 1:1 mixture of starting material to desired product by ¹H NMR spectroscopy. After purifying by flash column chromatography, the desired product was isolated as a white solid (113 mg, 44%). HRMS (ESI): C₃₆H₄₁N₂PbS₄ ⁺, 837.1913. Found, 837.1904.

4,4′-dimethyl-3λ⁴,3′λ⁴-2,2′-spirobi[[1,3,2]oxazaplumbolo[5,4,3-ij]quinoline] (2a). A round-bottom flask equipped with a stir bar was charged with H_(O ()9 mL), MeOH (9 mL), and NaOH (52.1 mg, 1.30 mmol). 8-Hydroxyquinaldine (100 mg, 0.628 mmol) was added and stirred to dissolve. Lead(II) acetate trihydrate (119 mg, 0.314 mmol) was added and stirred for 25 minutes. A yellow precipitate formed and the solution was filtered, and then washed with H₂O (about 20 mL) to yield a yellow solid (154.3 mg, 95%). HRMS (EI): Calcd for C₂₀H₁₆N₂O₂Pb⁺, 524.0973. Found, 524.0972.

3λ⁴,3λk⁴-2,2′-spirobi[[1,3,2]oxazaplumbolo[5,4,3-ij]quinoline] (2b). A round-bottom flask equipped with a stir bar was charged with H₂O (9 mL), MeOH (9 mL), and NaOH (54.5 mg, 1.36 mmol). 8-Hydroxyquinoline (90.8 mg, 0.626 mmol) was added and stirred to dissolve. Lead(II) acetate trihydrate (119 mg, 0.314 mmol) was added and stirred for 20 minutes. A yellow precipitate formed and the solution was filtered, and then washed with H₂O (about 20 mL), to yield a yellow solid (151.7 mg, 98%). HRMS (EI): Calcd for C₁₈H₁₂N₂O₂Pb⁺, 496.0660. Found, 496.0661.

C. Synthesis of Dithiocarbamate Salt

Sodium dibenzylcarbamodithioate (3). Sodium hydroxide (63.2 mg, 1.58 mmol) was dissolved in H₂O (0.3 mL) and MeOH (6 mL) in a round-bottom flask equipped with a stirbar. The solution was cooled to 0° C. and dibenzylamine (300 μ.L, 1.56 mmol) was added. The clear solution stirred vigorously for about 20 min at 0 ° C. before carbon disulfide (100 μL, 1.66 mmol) was added. The yellow solution was allowed to come to room temperature. The solvent was removed via rotary evaporation, yielding a white/yellow solid containing some water/methanol (485 mg, 105% with residual water). HRMS (EI): C₁₅H₁₄NS₂ ⁻, 272.0573. Found, 272.0574. 

1. A kit for determining a presence or absence of lead in a solid sample comprising: (a) a solution comprising: (i) a compound capable of forming a gel upon contact with lead, and (ii) a nonaqueous solvent; (b) an optional transparent container for the solution; and (c) an optional insert comprising instructions for use of the kit, wherein the compound has a structure

wherein

wherein n is an integer 1 or 2; m is an integer 1 or 2; and Y is an element selected from Groups IA and IIA of the periodic table.
 2. The kit of claim 1 comprising a transparent container.
 3. The kit of claim 1 or 2 wherein the container has a sealable-resealable cap.
 4. A kit for determining the presence or absence of lead in a solid sample comprising: (a) a compound capable of forming a gel upon contact with lead; and (b) an insert comprising instructions for use of the dithiocarbamate to determine the presence of absence of lead in the sample wherein the compound has a structure

wherein n is an integer 1 or 2; m is an integer 1 or 2; and Y is an element selected from Groups IA and IIA of the periodic table.
 5. The kit of claim 4 wherein the compound is provided as a solid.
 6. The kit of claim 4 wherein the compound is provided as a nonaqueous solution.
 7. The kit of claim 1 wherein the nonaqueous solvent comprises an alcohol, an ester, a glycol, a glycol ether, an aliphatic hydrocarbon, an aromatic hydrocarbon, a chlorinated solvent, and mixtures thereof.
 8. The kit of claim 1 wherein the nonaqueous solvent comprises a C₁₋₆ alcohol, a monoC₁₋₄alkyl ether of ethylene glycol or propylene glycol, acetone, methyl ethyl ketone, isophorone, dichloromethane, chloroform, ethyl acetate, 2-methoxyethanol, DMF, DMSO, THF, acetonitrile, kerosene, mineral spirits, xylene, toluene, turpentine, paint thinner, and mixtures thereof.
 9. The kit of claim 7 wherein the nonaqueous solvent further comprises up to 50%, by volume, water.
 10. A method of determining a presence or absence of lead in a solid sample comprising: (a) providing a nonaqueous solution of a compound capable of forming a gel upon contact with lead; (b) providing a test amount of a solid sample of interest; (c) contacting the nonaqueous solution of (a) with the solid sample test amount of (b) and observing the resulting mixture for the formation of a gel, wherein the compound has a structure

wherein n is an integer 1 or 2; m is an integer 1 or 2; and Y is an element selected from Groups IA and IIA of the periodic table.
 11. The method of claim 10 performed at a temperature of 20° C. to 50° C.
 12. The method of claim 10 performed at a temperature of greater than 20° C. to 30° C.
 13. The method of claim 10 wherein the nonaqueous solvent further comprises up to 50%, by volume, water.
 14. The method of claim 10 wherein the solid sample contains 200 ppm or more of lead. 